This invention relates to a system and method for single sideband modulation.
Single sideband radio transmission requires half the bandwidth of double sideband transmission, and is therefore economically attractive. However, neither of the two existing approaches to single sideband modulation are entirely satisfactory.
The "filter method" of modulation first uses a balanced modulator to produce double sideband, suppressed carrier modulation. The resulting signal is then filtered to pass only one of the two sidebands. This requires filters with a high rate of attenuation with frequency, if low frequency components of the modulating signal are to be preserved. Such filters are quite expensive.
The phasing method of single sideband modulation uses a trigonometric relationship, wherein quadrature components of the modulating signal and carrier are developed. One quadrature component of the modulating signal modulates a quadrature component of the carrier, to produce a first double sideband, suppressed carrier modulated output. The other quadrature component of the modulating signal modulates the second quadrature component of the carrier frequency to produce a second double sideband, suppressed carrier modulated output. The two double sideband outputs are then subtracted or added to produce the upper or lower sideband. To produce the quadrature components of the modulating signal requires precise, expensive networks. These networks do not produce an exact quadrature (90.degree.) phase shift over a wide bandwidth; therefore, they do not precisely balance out the unwanted sideband. Both this method and the filter method have substantial shortcomings as regards low frequency response and achieveable bandwidth.